1. Field of the Invention
The present invention relates to an image pickup device which photoelectrically converts incident light to produce signal charges, and more particularly to an image pickup device in which color mixture can be reduced, a method of producing such an image pickup device, and a semiconductor substrate for such an image pickup device.
2. Background Art
As shown in ITE Technical Report vol. 30, No. 25, pp. 25-28 “Rimen Shosya CMbS Image Sensor,” U.S. Pat. No. 5,828,088, Japanese Patent No. 3,722,367 and JP-A-2005-150463 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), a solid-state image pickup device of the backside illumination type, which has been used only in a specific application field where emphasis is placed on the quantum efficiency or the aperture ratio, has been proposed to be used in a consumer image pickup device such as a digital camera or a digital video camera.
The greatest feature of a backside illumination image pickup device is that the optical sensitivity can be improved by increasing the quantum efficiency. This is because, unlike the surface incident configuration, the rear face incident configuration has no structure for blocking incident light, on the light incident side, so that the aperture ratio can be increased, and a photoelectric converting portion can be designed to be sufficiently thick, whereby substantially 100% of incident light can be photoelectrically converted.
FIG. 8 is a graph showing the absorptance to the wavelength of incident light with respect to various thicknesses of a depleted layer of a photodiode which performs photoelectric conversion. As shown in FIG. 8, in a photodiode in which the depleted layer has a thickness of 2.0 μm, 3.5 μm, or 5 μm, the light absorptance in the photodiode is more reduced as the wavelength is longer. Therefore, it is seen that, when the thickness of the depleted layer of a photodiode is set to 10 μm, the reduction of the absorptance depending on the wavelength of incident light can be suppressed. In this way, when the thickness of the photodiode is increased, the reduction of the absorptance of incident light can be suppressed, and the optical sensitivity can be improved.
In the case where the thickness of a photoelectric converting layer is made large, there arise the following problems when an image pickup device is produced.
(1) In a photoelectric converting layer, impurity diffusion layers of different impurity concentrations are formed by an ion implanting apparatus. When the photoelectric converting layer is thick, the apparatus has a limit to implant ions to a sufficient depth.(2) In implantation of ions, when the ions are implanted to a deep region of the photoelectric converting layer, the range where the ions are implanted is wider as the depth is larger. Therefore, it is difficult to adequately form element separation for respective pixels.(3) In a production process in which optical elements are overlaid on a face of the photoelectric converting layer on the light incident side, and that on the side of a wiring board for transferring signal charges, a high overlay accuracy is required.
In a conventional backside illumination image pickup device, when a photoelectric converting portion is designed to be thick, therefore, there is a fear that color mixture is notably caused by so-called pixel crosstalk in which incident light enters photodiodes of adjacent pixels to generate signal charges.
In Non-patent Reference 1 above, a photodiode of the backside illumination CMOS sensor has a thickness of 4.0 μm, and the thickness is approximately equal to that of a conventional image pickup device. Therefore, the sensor does not employ the configuration which is an advantage of a backside illumination image pickup device, and in which a photoelectric converting layer can be made sufficiently thick, and hence pixel miniaturization by improving the optical sensitivity cannot be attained. At present, an image sensor having a pixel size of 2 μm square is commercially produced, and the development of a pixel of a smaller size is advancing. A request for a backside illumination image pickup device in which a minute pixel having a size of 1.4 to 1.6 μm square can be realized is increasing. In the configuration of an image pickup device in which the pixel size is smaller than about 1.5 μm and a silicon layer having a photoelectric converting layer of a thickness of 10 μm or more is disposed, the ratio of the width and thickness of the photoelectric converting layer of each pixel is 1:6 or larger. Usually, it is known that, when the thickness ratio is larger than 1, color mixture caused by crosstalk easily occurs. Non-patent Reference 1 above relates to the configuration where the thickness of the photoelectric converting layer is not increased, and which cannot solve the problem that color mixture occurs more easily as the ratio of the thickness to width of the photoelectric converting layer becomes larger.